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18.4.1.6 Making a Test Case If You Experience Table Corruption »
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18.4.1.5. Using Server Logs to Find Causes of Errors in mysqld

Note that before starting mysqld with --log you should check all your tables with myisamchk. See Chapter 5, MySQL Server Administration.

If mysqld dies or hangs, you should start mysqld with --log. When mysqld dies again, you can examine the end of the log file for the query that killed mysqld.

If you are using --log without a file name, the log is stored in the database directory as host_name.log In most cases it is the last query in the log file that killed mysqld, but if possible you should verify this by restarting mysqld and executing the found query from the mysql command-line tools. If this works, you should also test all complicated queries that didn't complete.

You can also try the command EXPLAIN on all SELECT statements that takes a long time to ensure that mysqld is using indexes properly. See Section 12.7.2, “EXPLAIN Syntax”.

You can find the queries that take a long time to execute by starting mysqld with --log-slow-queries. See Section 5.3.5, “The Slow Query Log”.

If you find the text mysqld restarted in the error log file (normally named hostname.err) you probably have found a query that causes mysqld to fail. If this happens, you should check all your tables with myisamchk (see Chapter 5, MySQL Server Administration), and test the queries in the MySQL log files to see whether one fails. If you find such a query, try first upgrading to the newest MySQL version. If this doesn't help and you can't find anything in the mysql mail archive, you should report the bug to a MySQL mailing list. The mailing lists are described at http://lists.mysql.com/, which also has links to online list archives.

If you have started mysqld with --myisam-recover, MySQL automatically checks and tries to repair MyISAM tables if they are marked as 'not closed properly' or 'crashed'. If this happens, MySQL writes an entry in the hostname.err file 'Warning: Checking table ...' which is followed by Warning: Repairing table if the table needs to be repaired. If you get a lot of these errors, without mysqld having died unexpectedly just before, then something is wrong and needs to be investigated further. See Section 5.1.2, “Server Command Options”.

It is not a good sign if mysqld did die unexpectedly, but in this case, you should not investigate the Checking table... messages, but instead try to find out why mysqld died.